A proton exchange membrane fuel cell (PEMFC) is a device for converting chemical energy of fuel (fuel gas) to electrical energy. A PEMFC implements a solid polymer as an electrolyte membrane (an ion exchange membrane). An anode is disposed on one side of the membrane, and a cathode is disposed on the other side of the membrane. Generally, an assembly including the membrane, the anode, and the cathode is called an MEA (membrane and electrode assembly).
A general operation temperature of the PEMFC is less than 100 degrees Celsius, and the PEMFC is generally used as power source of a vehicle or as a small electric power source.
In the PEMFC, hydrogen is supplied to the anode (fuel electrode), and oxidation gas (air) is supplied to the cathode (air electrode). Hydrogen that is supplied to the anode is resolved into hydrogen ions H+ and electrons e− by a catalyst. The hydrogen ions are transmitted to the cathode through the membrane, and the electrons are transmitted to the cathode through a separation plate, which is made of a conductive material. The hydrogen ions and the electrons that are supplied to the cathode react with oxidation gas supplied to the cathode to generate water. At this time, electron flow from the anode to the cathode generates a current, and heat and water are also generated during the reaction.
Such a reaction in the PEMFC is as follows:
[Reaction in the anode]2H2→4H++4e−
[Reaction in the cathode]O2+4H++4e−→2H2O
[Whole reaction]2H2+O2→2H2O+electric energy+thermal energy
In the PEMFC, for a smooth flow of hydrogen ions from the anode to the cathode, sufficient moisture must be supplied to the membrane through which the hydrogen ions flow. A device to supply moisture to the membrane is generally called a humidifier, and various kinds of humidifiers are being developed.
When the membrane is not sufficiently humidified, moisture on a surface of the membrane is evaporated by gases supplied to the membrane. Accordingly, the membrane lacks moisture for the transmission of the hydrogen ions, and the hydrogen ions are not transmitted. This deteriorates an electro-chemical reaction in the fuel cell.
On the other hand, when excessive water is supplied by the humidifier, or when the fuel cell operates under a high output so that excessive water is generated in the fuel cell, excessive water may be supplied to the membrane. When water is excessively supplied to the membrane, the catalyst reacting the hydrogen ions, the electrons, and the oxidation gas is surrounded by the water. Accordingly, access of the hydrogen ions and the oxidation gas to the catalyst is hindered, so that the output of the fuel cell drops.
Therefore, in order to operate the PEMFC at optimal efficiency, supplied moisture and generated water must be considered, based on operating conditions, to regulate the amount of water supplied by the humidifier.
A conventional humidifier used for the PEMFC humidifies the hydrogen and the oxidation gas with fuel cell stack cooling water that circulates in the fuel cell stack. By using such an integrated humidifier, the volume of the fuel cell system can be decreased. However, safety cannot be assured when there is an assembling defect or an outside shock. Furthermore, in the PEMFC using the conventional humidifier, water for humidification is supplied from the fuel cell stack cooling water that is composed of de-ionized water (DI water). Therefore, the humidifier of the PEMFC cannot operate below the freezing point.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.